Droplet mist generator

ABSTRACT

In a pump chamber connected to a liquid supply, an overlapping piezoelectric flexural transducer is disposed so that when voltage pulses are applied to produce an excursion, a number of droplets can be expelled from a nozzle array in the housing wall of the pump chamber using a plurality of nozzles. Gaps are formed between the edges lateral to the direction of overhang an the free end of the piezoelectric flexural transducer ad adjacent section of the housing wall. The nozzle array can be disposed in the projection of the plate surface of the piezoelectric flexural transducer in its direction of motion or in the extension of the piezoelectric flexural element or in another suitable pattern. As part of a combustion device the droplet mist generator is excellent for producing a combustible fuel-oxidant mixture.

This application is a 35 U.S.C. 371 of application numberPCT/DE97/010307 filed Jun. 24, 1997.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention concerns a droplet mist generator and, in particular, adroplet mist generator as apart of a burner.

2. Summary of the Invention

Micro-droplet mist generators for producing individual droplets on callare known in ink printing. In EP-O 713 773 a droplet mist generator withpiezoelectric flexural transducers and a nozzle each under thetransducer is proposed in which the individual transducers withpartition walls are separated from each other so that when thetransducer is deflected from the true path, a droplet is ejected fromthe nozzle assigned to another transducer.

From the older German patent application with the file number 19507978.7a dosing system for fuel dosing is known that has numerous micro-nozzlesand electrothermic, electrostatic, electrodynamic, or piezoelectrictransducers with which an expansion of vapor bubbles in a fuel-filledchamber or a change in volume of this chamber is effected by means of anelectrical trigger signal, therefore making it suitable for the repeatedejection of fuel droplets that are essentially of the same size. The useof a piezoelectric membrane actuator is described as a preferredtransducer principle.

When using the expansion of vapor bubbles as an actuator principle fordosing traditional types of fuel, the various components of the fuelvaporize under very different conditions. The vaporization thereforedoes not occur abruptly enough to achieve an efficient formation ofdroplets. Variations in the composition of the fuel lead, in addition,to irregularities so that reliable dosing or transport is not possiblewhen using the vapor bubbles principle. Transducers in which the chambervolume is changed are complicated structures. In the case of apiezoelectric flexural transducer, for example, a piezoelectric ceramicelement is covered with a membrane that forms a chamber wall. This isnecessary to obtain the change in volume, because when a piezoelectriccrystal expands in a direction, there is always a vertical contractionconnected with it. In the piezoelectric flexural transducer and themembrane, material must be deformed during a large-scale deflection fromthe true path so that works of deformation must be carried out againststrong inner mechanical resistance. Such transducers therefore work witha poor degree of effectiveness. And in relation to the structural sizeof the transducer elements, only a small dispersion is attained due tothe resistance. A high acceleration of fluid also cannot be obtained.

By using the invention, the problem of creating an inexpensive pump witha small structural size in which a stream of fluid in the form of acloud of droplets can be dosed with a high flow rate while maintaining acertain droplet size and density is solved.

The problem is solved according to the invention by a droplet mistgenerator. The droplet mist generator comprises a pump chamber, which isconstructed in a casing and is connected to a fluid reservoir; a nozzlearea constructed in the casing wall, having a plurality of nozzles; aplate-shaped piezoelectric flexural transducer that is positioned in thepump chamber and attached so that it overhangs and can be bent around aquadrature axis running transversely to the direction of the overhang;openings that are constructed between the edges of the piezoelectricflexural transducer, which form ends in the direction of its quadratureaxis, and the casing wall; and a control system through which voltageimpulses can be applied to the piezoelectric flexural transducer bybending the piezoelectric flexural transducer, driving out fluids, andejecting droplets from the nozzles of the nozzle area.

With the idea of impacting an entire area of nozzles with apiezoelectric flexural transducer positioned so it is effectivelyfluidic inside a chamber filled with fluid, a droplet mist generatorwith an especially high flow rate is created, whereby the droplet sizeand density can be determined with the form of the nozzle area and bymeans of the length, strength, and frequency of the pulse emitted by thecontrol system.

Piezoelectric flexural transducers produce an especially high deflectionfrom the true path when accelerating quickly and can be operated withhigh frequencies. In addition, they have only a small inner mechanicalresistance. Using the piezoelectric flexural transducer principle, ahigh conversion rate of electrical to mechanical energy can be obtainedwith respect to the structural size. Moreover, piezoelectric flexuraltransducers are simple constructions and thus are inexpensive andreliable.

The special arrangement of the transducer and the numerous nozzles leadsto the fact that the transformed mechanical energy can be used for theproduction and transport of the droplet stream with a high degree ofefficiency. By transforming the energy directly near the nozzles onwhich the droplets are formed, a high share of fluidic energy issupplied for the formation of droplets and their transport.

The fluidic losses due to the compression of the fluid are, moreover,minimized because the transformer surface, in front of which a peakpressure is produced during the impacting action, with the nozzle areasfaces a large nozzle cross-sectional area, through which a conversion ofthe produced pressure takes place during transport by forming andejecting droplets. In other words, a large share of the generatedpressure is transformed.

Through the high acceleration of the piezoelectric flexural transducerthe entire energy is supplied to the droplets forming on the nozzle inthe shortest time span, which leads to an abrupt breaking off of thedroplets while preventing a larger back-flow into the chamber.

The opening between the edges of the piezoelectric flexural transducerand the casing wall allows the fluid to stream around the piezoelectricflexural transducer during the backward movement of the piezoelectricflexural transducer so that the increasing volumes between thepiezoelectric flexural transducer and the nozzle area are filled withthe fluid that is flowing back and no air is pulled into the nozzles inthe chamber. The openings are therefore calculated to be so large thatfluidic resistance that occurs due to friction remains small enough thatthe deflection from the true path is not greatly impaired. At the sametime, the openings are calculated so they are so small that during therapid impacting action of the piezoelectric flexural transducer thefluid located in front of the transducer cannot be carried off quicklyenough through the opening and is pushed through the nozzles.

The voltage pulses given off by the control system are coordinated insuch a way that the transport of fluid is made possible. The impactingaction, which causes the ejection of droplets through the nozzle, canoccur considerably more quickly than the backward movement of thepiezoelectric flexural transducer so that during the impacting action nostreaming occurs through the opening in which the backward flow runsagainst a sufficiently strong stream. For the purposes of the presentinvention, a known control system can be used.

By using a single piezoelectric flexural transducer to impact severalnozzles, the system is inexpensive and not very prone to problems.

According to the invention the chamber and fluid reserve can beconnected to any suitable place in the chamber. Preferred, however, is aconnecting line on one of the sides of the piezoelectric flexuraltransducer turned away from the nozzle area. If one does not completelyreduce the volume of the chamber, but reduces the volume between thepiezoelectric flexural transducer and the nozzles, when the volume onthe opposite side is raised, fluid can be drawn from the fluid reserveconnected to the pump chamber while the droplets are ejected. In sodoing one can obtain especially short repeat times between thesuccessive surges or bending and droplet-ejection operations, as aresult of which the transport performance is raised even more.

According to the invention the chamber can be connected to the fluidreserve by means of a line or other connection. Preferably, however, thechamber is connected to the fluid reserve through several lines,especially two lines. In so doing, the droplet mist generator can bedegassed during operation by providing fluid through a connecting lineand carrying away gas and fluid through the other connection lines.Moreover, an improved and quicker fluid feed can be obtained with amajority of lines, each in a suitable arrangement, which leads to ashortened refill time between two droplet-producing pulses.

According to the invention the connections between the chamber and fluidreserve can be designed so there is as little resistance as possible.Preferred are, however, choke sites in the connections that provide thatthe least possible fluid is driven through the feed lines that connectthe chamber with the fluid reserve, thus guaranteeing that the transportperformance of the droplet mist generator is high. Preferably the chokesites are designed in such a way that the fluid goes against a highfluidic resistance during a high pressure impulse when a droplet isejected, while with a small difference in pressure during the refilloperation the fluid goes against only a small fluidic resistance andthus the spray frequency can be increased. Flap valves can also beprovided in the connections so that a streaming of fluid into thechamber through the connection is made possible while at the same timepreventing the fluid from streaming out.

According to the invention the nozzles can be designed ascylinder-shaped channels, openings, channels with square cross-sectionalareas, or channels of any other shape; and they can have a constantchannel cross section. They can also be designed so they taper towardthe chamber. It is, however, preferable that they are designed so theytaper in the direction away from the chamber. In so doing, thecross-sectional area of the nozzle with the smallest diameter isobtained on the opening of the nozzles in the surrounding environment.Because bordering surfaces between two fluids constantly strive to takeon the state with the least energy in the smallest area of the boundarysurface, a nozzle tapering outward leads to a situation in which theedge of the meniscus between the fluid and gaseous environmentconstantly strives to remain on the outer edge of the nozzle. Byreducing the extent of the change in the position of the meniscus edge,the droplet mist generator is guaranteed to work in an especially robustway, which leads to a higher transport performance because no outfallcycles result.

According to the invention, the outer side of the casing wall in thepart of the casing wall in which the nozzle field is positioned can bemade of any suitable material. Preferred, however, is a coating withteflon or with another suitable anti-adhesive material. With such acoating one prevents the outer side from being moistened, i.e., a movingforward of the 3-phase boundary between fluid, gaseous surroundings andthe casing structure results from opening the nozzle. As a consequence,the meniscus edge remains at the end of the nozzle toward the outsideduring the formation of the droplets, as a result of which the inventionis guaranteed to work in a robust fashion with a high transportperformance.

According to the invention the droplet mist generator can have anysuitable piezoelectric flexural transducer. Preferably, however, thepiezoelectric flexural transducer is a multiple-layer piezoelectricceramic transducer with an additional passive piezoelectric layer. In sodoing, the same deflection of the piezoelectric flexural transducer canbe obtained with a small control voltage. This has the advantage thatthe regulations for the maximum voltage can be observed with manypossible uses of the droplet mist generator without limiting theproductivity.

According to the invention the droplet mist generator can have only onepiezoelectric flexural transducer and only one nozzle area. According tothe invention a majority of piezoelectric flexural converters and/or amajority of nozzle areas can be provided in the droplet mist generator.In this connection several piezoelectric flexural transducers arearranged in such a way that their plate surfaces can be positioned in aplane next to one another or their plate surfaces can be positioned invarious levels so they overlap each other or are positioned next to eachother. In a preferred form of the model an arrangement is provided witha second piezoelectric flexural transducer and a second nozzle area thatlie across from the free end of the first piezoelectric flexuraltransducer and that are essentially mirror-inverted to the firstpiezoelectric flexural transducer and the first nozzle area. The controlsystem in this case is constructed in such a way that the piezoelectricflexural transducer and the second piezoelectric flexural transducer canbe controlled by various pulse frequencies, pulse length, and/or pulsephases. The arrangement of the two piezoelectric flexural transducerslying across from one another with the same control of the piezoelectricflexural transducers leads to a situation in which the fluid, which isdriven out to the other piezoelectric flexural transducer, is subject tofluidic resistance due to the incoming fluid forced out of the otherpiezoelectric flexural transducer. As a result, a higher pressure canbuild up and the transport flow rate can be increased. By using acontrol with shifted pulse phase the transport flow rate can be varied.A control can also be carried out with various pulse frequencies and/orpulse lengths. A variation or different control with respect to one ormore of the parameters pulse frequency, pulse length, and pulse phasecan also be used with a set nozzle arrangement in the nozzle area tovary the droplet size and droplet speed.

According to the invention the nozzle area can be designed in anysuitable part of the casing wall. In an especially preferred form thenozzle area is designed in a part of the casing wall that is positionedinside the overhang of the plate surface of the piezoelectric flexuraltransducer in the direction in which the free end of the piezoelectricflexural transducer is movable when passing through its equilibriumposition. The nozzles of the nozzle area are thus essentially positionedin such a way that all the nozzles would be covered by the transducersurface if one would move the piezoelectric flexural transducer up tothe part of the casing wall in which the nozzles are constructed. Inthis working model an opening of a suitable size is designed between thefree end of the piezoelectric flexural transducer and the part of thecasing wall lying across from it in the extension of the transducer.

According to the invention any suitable distance or no distance at allmay separate the piezoelectric flexural transducer from the part of thecasing wall in which the nozzle area is designed. In a preferred form ofthe model when the piezoelectric flexural transducer is in itsequilibrium position, a small distance between the piezoelectricflexural transducer and the part of the casing wall in which the nozzlearea is designed is formed. In this case the piezoelectric flexuraltransducer can be moved away from the nozzle area by applying a voltagepulse and then moved back to the nozzle area by applying a reversepolarized voltage or using mechanical restoring forces, whereby thedroplet ejection is effected. If the distance is chosen to be smallenough, overshooting the equilibrium position when moving it back canlead to a situation in which the piezoelectric flexural transducer hitsagainst the casing wall in which the nozzle area is constructed. Thepiezoelectric flexural element can, however, be moved away by applyingthe voltage pulse immediately in the direction toward the nozzle area sothat the droplet ejection can be started directly when applying thevoltage pulse. In this case as well the piezoelectric element hitsagainst the casing wall. This bumping against the casing wall can havethe advantageous effect that the acceleration of fluid is quite abruptlybroken off, resulting in an especially regular and quick break off ofthe droplets. How strong this effect is can depend upon how thepiezoelectric flexural transducer and the part of the casing wall inwhich the nozzle area is constructed are formed. If there are planesurfaces, contact will occur to a great extent across the entiresurface; if there are arched surfaces or non-plane surfaces shaped inanother form, contact occurs only at one or a few places.

The opening between the free end of the piezoelectric flexuraltransformer and the casing wall lying opposite it in the extension ofthe piezoelectric flexural transformer can have any width according tothe invention. Preferably, however, it is not more than five times aslarge as the gap that occurs when the piezoelectric flexural transformeris in equilibrium position when no voltage is applied.

In another preferred form of the model the piezoelectric flexuraltransformer in its equilibrium position, which occurs when no voltage isapplied, lies on the part of the casing wall in which the nozzle area isconstructed and the piezoelectric flexural transformer is moved awayfrom the nozzle field by applying voltage by using the control system.In this case the formation of droplets is triggered when thepiezoelectric flexural transducer springs back after the voltage pulseends by applying a reverse voltage impulse or mechanical restoringforce.

According to the invention the part of the casing wall in which thenozzle area is constructed can be constructed like the other parts ofthe casing wall. Preferably the part of the casing wall nonethelessprojects into the chamber. Such a form has the advantage that highpressure, which builds up in the gap that becomes more and more narrowas the surface of the piezoelectric flexural transducer is moved to thecasing wall, builds up only in the area in which it falls when thedroplets emerge from the nozzles and thus can be utilized. As a result,there is a reduction of the fluidic losses during the droplet ejectionoperation and thus an increase of the transport performance and theefficiency of the pump. An advantageous effect is also obtained when thefluid is refilled from the reservoir. The narrow distance between thepiezoelectric flexural transformer and the casing wall, in which fluidcan only flow against a high fluidic resistance, is shorter compared toa form of the model without casing wall parts designed to project intothe chamber. As a consequence, the necessary fluid can be drawn backmore quickly and the droplet production frequency and the transportquantity can be further increased.

In another preferred form of the model the nozzle area is positioned soit lies across from the free end of the piezoelectric flexuraltransformer in the extension of the piezoelectric flexural transformer.In this way the nozzle area is staggered a little bit with respect tothe free end of the piezoelectric flexural transformer. The nozzles arethus, preferably, in the direction of overhang of the piezoelectricflexural transformer. Such an arrangement has the advantage that it ispossible, given an especially small construction size, to arrange amajority of the piezoelectric flexural transformers in the direction ofthe plate surface one after the other or inside the plate surface planenext to each other, whereby each piezoelectric flexural transformer canbe assigned to a corresponding nozzle area without having to furtherenlarge the construction area required to set up the piezoelectricflexural transformer due to the nozzle area. Preferably, in thisarrangement when the piezoelectric flexural transformer is in itsequilibrium position, there is a gap between the piezoelectric flexuraltransformer and the next wall lying vertical to the plate surface of thepiezoelectric flexural transformer.

According to the invention the droplet mist generator is a droplet mistgenerator for any suitable fluids. In this connection the droplet mistgenerator according to the invention can be used separately or as acomponent of any suitable system. Preferably, the droplet mist generatoris nonetheless a component of a burner, whereby the fluid reserve is afluid fuel reserve. The nozzles of the nozzle area serving as burnernozzles have a smallest diameter of at least 10 μm and at most 100 μm.As a result, droplet sizes are obtained that are especially well-suitedfor the production of an inflammable mixture made of fuel droplets and agaseous oxidant. With traditional fluid fuels such as diesel fuel orgasoline, such droplet sizes lead to a situation in which the fueldroplets completely evaporate right after the ejection from the nozzle,resulting in an inflammable and/or highly combustible mixture. Dependingon the viscosity and transport quantity, the nozzles according to theinvention have diameters larger than 100 μm corresponding to the fluidicrequirements.

According to the invention the mid-points of each of the neighboringnozzles of the nozzle area that serve as burner nozzles have anysuitable distance between them. Preferably, the mid-points nonethelessoccur at intervals of at least 50 μm and at most 2,000 μm. By choosingthe distances between the neighboring nozzles in this arrangement oneobtains a further improvement of the fuel-oxidant mixture, and with it afurther increase in the burner performance.

According to the invention the droplet mist generator can have anynumber of nozzles depending on its use. Preferably, however, the dropletmist generator has at least 50 nozzles. With at least 50 nozzles or morea burner is especially well suited for use as a burner for vehicleheating or household heating devices.

In another preferred form of the model holes are provided in thepiezoelectric flexural transducer according to the invention to reducethe fluidic resistance of the piezoelectric flexural transducer. In yetother forms of the model valves according to the invention can beprovided in the droplet mist generator with which the transport of fluidis possible even with larger nozzle diameters. In this connection theinvention provides that either droplets or a continuous stream of fluidis transported. Preferably, the operation of existing valves is carriedout with a piezoelectric flexural transducer that simultaneouslyconverts the fluidic energy. According to the invention the chamber onthe nozzles can also be sealed off against its surroundings by bringingthe piezoelectric flexural transducer into a certain position.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantageous forms of the invention are described in connection with thedrawing. The following are shown in the drawings.

FIG. 1a shows a sectional view transverse to the direction of overhangof the piezoelectric flexural transducer of a droplet mist generator inaccordance with a working form of the invention, whereby thepiezoelectric flexural transducer is in its equilibrium position.

FIG. 1b is a sectional view of the droplet mist generator in accordancewith FIG. 1a, whereby the piezoelectric flexural transducer is deflectedby applied voltage.

FIG. 1c is a sectional view of the droplet mist generator from FIG. 1aalong the dotted line drawn in in FIG. 1b.

FIG. 2a is a sectional view of a droplet mist generator according toanother model of the invention in which the part of the casing wall inwhich the nozzle area is constructed projects into the chamber, wherebythe piezoelectric flexural transducer is in its equilibrium position.

FIG. 2b is a sectional view of the droplet mist generator according toFIG. 2a, whereby the piezoelectric flexural transducer is deflected byapplied voltage.

FIGS. 3, 4, and 5 are all sectional views of a droplet mist generator inaccordance with another working form of the model.

FIG. 6 is a sectional view of a droplet mist generator in accordancewith yet another working form of the model in which two arrangementsfrom a piezoelectric flexural transducer and a nozzle area face eachother in mirror-inverted fashion with respect to the free end of thepiezoelectric flexural transducer.

FIG. 7 is a sectional view of a droplet mist generator in accordancewith yet another working form of the model in which the nozzle area ispositioned opposite its free end lying in the extension of thepiezoelectric flexural transducer.

FIGS. 8, 9, 10, 11, 12 are all sectional views of a droplet mistgenerator in accordance with yet another working form of the model inwhich the nozzle area is positioned opposite the free end lying in theextension of the piezoelectric flexural transducer.

FIG. 13a is a sectional view of a nozzle area designed according to theinvention.

FIG. 13b is a top view onto the nozzle area designed according to theinvention and represented in FIG. 13a.

FIG. 14 is a view of a droplet mist generator from FIG. 9 in a top viewin the direction vertical to the plate surface of the piezoelectricflexural element.

FIG. 15 is a representation of an example of the contact of apiezoelectric flexural transducer in a droplet mist generator designedaccording to the invention.

FIG. 16 is a principal representation of a bimorph piezoelectricflexural transducer.

FIG. 17 is a principal representation of a monomorph piezoelectricflexural transducer.

FIG. 18 is a principal representation of a multi-layer piezoelectricflexural transducer.

And FIG. 19 is a principal representation of a control system used inaccordance with a working form of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIGS. 1a to 1c one can see a construction of a droplet mist generatoraccording to an advantageous working form of the invention. In a casinga pump chamber 1 is constructed that can be filled with fluids. Thecasing wall 2 is formed by a casing base part 2c, a casing middle part2b, and a casing top part 2d. Inside the chamber 1 a piezoelectricflexural transducer 4, which can be deflected from its true path by thecontrol system 6 (not shown in FIGS. 1a-1c), is attached so itoverhangs. As can be seen in FIGS. 1a and 1c, the piezoelectric flexuraltransducer 4 is designed in a plate shape. Its end 4e is attached insidethe casing. The opposite end 4d is free. The plate surface 4c is boundedby the edges 4b positioned on the sides in the direction of theoverhang. The piezoelectric flexural transducer 4 is made of two layers4f, 4g of piezoelectric ceramic. By applying voltage, the piezoelectricflexural transducer 4 can be bent around the axis 4a running transverseto the direction of overhang. With such bending, as can be seen in FIG.1b, the free end 4d moves along a curve, which, by way of approximation,corresponds to a movement vertical to the direction of overhang and tothe neutral axis 4a.

A part 2a of the casing wall 2 is positioned inside the overhang of theplate 4c on the casing wall 2 in the direction of the movement of thefree end 4d of the piezoelectric flexural transducer 4 when it passesthrough its equilibrium position on the neighboring part of the casingwall. A nozzle area 3 with a majority of nozzles 3a is constructed inthe part 2a of the casing wall 2. In the working example shown here theplate surface 4c and the part 2a of the casing wall 2 are even surfacesthat run parallel to each other.

As can be seen in FIG. 1a, when the piezoelectric flexural transducer 4,is in equilibrium position, which occurs when the voltage is off, a gap7 forms between the piezoelectric flexural transducer 4 and the part 2aof the casing wall 2 in which the nozzle area 3 is formed.

As one can see in FIG. 1c, between the edges 4b of the piezoelectricflexural transducer 4 and the casing wall 2 openings 5a are providedthat are dimensioned large enough so that a movement of thepiezoelectric flexural transducer 4 is not opposed by a flow resistancethat is too strong, and when the piezoelectric flexural transducer 4 ismoved back from the nozzle area 3 a sufficient current linkage can occurso that no air is drawn into the chamber 1 through the nozzles 3a. Atthe same time the openings 5a are sufficiently narrow so that whenmoving the piezoelectric flexural transducer 4 onto the nozzles 3a thefluid cannot go around the openings 5a quickly enough but instead isforced through the nozzles 3a.

Between the free end 4d of the piezoelectric flexural transducer and theopposite part of the casing wall lying in its extension an opening 5b isalso constructed that is less than 5 times as wide--namely, about 4times as wide--as the gap 7. In the working example seen in FIG. 1 thepiezoelectric flexural transducer has measurements of 9×4×0.5 mm. Theactive, free length is 5.5 mm. The deflections that can be obtained onthe free end are 25 μm at 50 V.

As one can see in FIG. 1, the chamber 1 on the side of the piezoelectricflexural transducer 4 turned away from the nozzle area 3 is built largerthan it is on the other side of the gap 7. When deflecting thepiezoelectric flexural transducer 4 from its true path, excessivelylarge changes in pressure do not occur in this part of the chamber 1.The casing middle part 2b of the casing wall 2, which is positionedbetween the casing base part 2c and the casing top part 2d and whichdetermines the height of the chamber, has a height of 675 μm in thisexample. Preferably, the casing components are made of silicon.

As is also clear from FIG. 1, the chamber 1 is connected through lines 8to a fluid reserve (not shown). Choke sites 8a are constructed in thelines 8. The lines 8 are at a considerable distance from each other.They can therefore also be used for rinsing during the operation of thepump. In this connection it is advantageous that one of the two lines 8is positioned at the end of the casing in the direction toward the freeend 4d of the piezoelectric flexural transducer 4. With a correspondingorientation of the chamber 1 relative to gravity, the pump can bedegassed by having the fluid flow through the line 8 positionedcentrally, with the outlet through the line 8 positioned at the end. Gasbubbles that appear rise to the top and are rinsed out of the chamber 1.When the pump is in operation, the arrangement shown in FIG. 1, whichhas several lines 8 that connect the chamber 1 to the fluid reserve, isalso advantageous. In the suction phase, evenly occurring drops inpressure occur by way of the chamber 1. The refill operation can thus becompleted more quickly when two lines 8 exist. In the working exampleshown in FIG. 1 the line 8 has an inner diameter of 1 mm.

By applying voltage impulses to the piezoelectric flexural transducer 4by using a control system 6, the piezoelectric flexural transducer isdeflected from its true course. In so doing, fluid can be driven ontothe nozzles and droplets ejected out the nozzles 3a. In the working formdescribed the piezoelectric flexural transducer 4 can be moved to andfrom the nozzle area 3 by applying voltage my means of the controlsystem 6. As can be seen in FIG. 1b, the piezoelectric flexuraltransducer 4 can be deflected so far by moving it from the nozzle area 3that the free end 4d of the piezoelectric flexural transducer 4 hitsagainst the part 2a of the casing wall in which the nozzle area 3 isconstructed. As a result, the movement of the piezoelectric flexuraltransducer 4 is abruptly slowed, which leads to a particularlyadvantageous breaking off of the droplets. To improve the dropletejection behavior the piezoelectric flexural transducer 4 can,nonetheless, first be moved a certain distance away from the nozzle area3 so that a greater amount of fluid exists between the piezoelectricflexural transducer 4 and the nozzle area 3 before the piezoelectricflexural transducer 4 is moved onto the nozzle area 3.

As one can see in FIG. 1, the piezoelectric flexural element consists oftwo layers 4f, 4g. They are connected to each other so they cannot beslid back and forth. From FIG. 17 one can see more clearly theconstruction of the piezoelectric element used in this working form ofthe invention. It is a monomorph actuator. One of the layers is made ofa piezoelectric ceramic layer; the other, of metal or another suitablematerial. Due to the piezoelectric effect, the piezoelectric ceramiclayer is extended or compressed by applying voltage. When extending orcompressing the layer with respect to the other layer, the layerconstruction is bent. This process can be reversed by discharging. Thiscan take place either by applying the corresponding countervoltage or bya slow, independent discharging process.

Other working forms of the piezoelectric flexural transducer accordingto the invention can be seen in FIG. 16 with a bimorph piezoelectricactuator and in FIG. 18 with a multi-layer piezoelectric flexuralactuator. In the bimorph actuators two piezoelectric ceramic plates areprovided with an electrode in the middle, as a result of which bothlayers are reverse polarized. By applying voltage the one layer isextended and the other compressed so that a larger bending occurs withequally applied differences in voltage. In a multi-layer piezoelectricflexural element the extensible or compressible layer is constructedfrom alternately very thin--e.g., 20 μm--piezoelectric layers andelectrodes stacked on each other, which are fused with each other orfirmly glued together. In this case the electrodes are interlocked as ina film capacitor--i.e., the inverse polarized electrodes alternate. As aresult the same electrical field strength is produced in thepiezoelectric ceramic layers with low voltage and thus the same extentof the piezoelectric effect is produced. The operating voltage fallsconsiderably in such a case, e.g., from several 100 V to about 30 to 60V.

As can be seen in FIG. 1, at least two nozzles 3a exist, which form thenozzle area 3.

In the FIGS. 13a and 13b one can see how the nozzles 3a and the nozzlearea 3 are formed in another advantageous working form. As is clear inFIG. 13a, the nozzles are designed in such a way that they taper fromthe chamber inner side to the chamber outer side. The part 2a of thecasing wall in which the nozzles 3a of the nozzle area are constructedhas a 35-μm thick teflon layer on the outside (not shown in thediagram).

In FIG. 13b the arrangement of the nozzles is shown in FIG. 13a in a topview. The nozzles are positioned regularly with an equal distancebetween neighboring nozzles. In each case the series of nozzles ispositioned so the nozzles are staggered with respect to a neighboringseries of nozzles. This allows for the possibility of packing thenozzles as closely as possible while taking into consideration technicalmanufacturing specifications.

Another advantageous working form of the droplet mist generatoraccording to the invention can be seen in FIGS. 2a and 2b. The part 2aof the casing wall 2 in which the nozzle area 3 is formed projects intothe chamber 1. The piezoelectric flexural transducer 4 lies inequilibrium position on the part 2a of the casing wall 2 in which thenozzle area 3 is formed. In the area neighboring on the nozzle area 3there is a gap 7 between the piezoelectric flexural transducer 4 and thecasing wall 2. While operating the droplet mist generator thepiezoelectric flexural transducer 4 is first moved from its equilibriumposition from the nozzle area and then moved back onto the nozzle area 3by either applying a reverse polarized voltage or mechanical restoringforces.

In FIG. 3 another working form of the droplet mist generator accordingto the invention can be seen. The casing is made of the three components2d, 2c, and 2e, which form the casing wall 2. In this connection thecasing base part 2c is designed as a plate. The piezoelectric flexuraltransducer 4 is squeezed in between the casing parts 2c and 2d andanchored in this way. In FIG. 15 one can see the construction of thecontact of the piezoelectric flexural transducer with the contactsprings 10a, 10b in this working example.

Another working form of a droplet mist generator according to theinvention can be seen in FIG. 4. The casing is made of only two casingparts, whereby the piezoelectric flexural transducer 4 is firmlysqueezed between the casing base part 2c and the casing top part 2dlying opposite it.

In FIG. 5 another working form of a droplet mist generator according tothe invention can be seen. As can be seen in the working form in FIG. 2,the part 2a of the casing wall 2 is formed so it projects into thechamber 1. In this case the piezoelectric flexural element 4, however,does not rest on the part 2a of the casing wall 2 in its equilibriumposition; rather, there is a gap between the piezoelectric flexuraltransducer 4 and the part 2a of the casing wall 2. The piezoelectricflexural element can therefore be bent directly onto the nozzle area sothat droplets are ejected by using the control system 6. If thepiezoelectric flexural element 4 in this working form is then moved awayfrom the nozzle area 3 by using the control system 6, advantages occurcompared to the working form represented in FIG. 2. The surfaces of thepiezoelectric flexural transducer 4 lying across from each other and thepart 2a of the casing wall 2 are already moistened with fluid when thepiezoelectric flexural transducer 4 is moved away from the part 2a ofthe casing wall, as a result of which fluid is drawn more quickly intothe larger-growing gap and a higher spray frequency is obtained.

Still another advantageous working form of a droplet mist generatoraccording to the invention can be seen in FIG. 6. Two piezoelectricflexural transducers 4 and two nozzle areas 3 lie across from each otherin mirror-inverted fashion.

Another advantageous working form of a droplet mist generator accordingto the invention can be seen in FIG. 7. The nozzle area 3 in this caseis formed in the extension of the piezoelectric flexural transducer 4across from the free end 4d of the piezoelectric flexural transducer inthe casing wall. In the working form that can be seen in FIG. 7 theentire length of the piezoelectric flexural transducer 4 lies againstthe casing wall 2, and the nozzle area 3 is formed in one of the cornersof the casing wall 2 lying across from one of the ends of thepiezoelectric flexural transducer 4. In this case the nozzle area isformed on the boundary surface between the two casing components--thecasing base part 2c and the casing top part 2d.

In two other advantageous working forms, which can be seen in FIGS. 8and 9, the entire length of the piezoelectric flexural transducer 4 doesnot lie against the casing wall 2 in its equilibrium position; itsattached end 4e is anchored onto the casing base part 2c of the casingwall 2, and in the area of the free end 4d of the piezoelectric flexuraltransducer 4 there are recesses 9 provided in the casing base part 2cthat are designed as grooves. With the grooves the space of the chamber1 is expanded on the side of the piezoelectric flexural transducerturned away from the lines 8, through which the chamber 1 is connectedto the fluid reserve. The recesses 9 in the casing base part 2cessentially extend in the direction of the overhang of the piezoelectricflexural transducer 4. In the corner of the chamber 1 formed in theplace of the casing wall 2 in which the casing base part 2c and thecasing top part 2d meet each other, the recesses 9 change over into thenozzles 3a of the nozzle area 3. In this corner the recesses 9 form thenozzles 3a in the casing wall alone or together with other partialrecesses in the casing top part 2d, as one can see in FIGS. 8 and 9.

In FIGS. 10, 11, and 12 working forms can be seen in which the pumpchamber 1 and the nozzles 3a are essentially designed as in the workingforms of FIGS. 7, 8, and 9. But the piezoelectric flexural transducer 4is not attached to only one casing component part 2c (as in FIGS. 7, 8,and 9), the piezoelectric flexural transducer 4 is attached to thecasing between the casing base part 2c and the casing top part 2d.

In FIG. 14 in a top view, recesses 9 provided are positioned as in theworking forms of the invention in FIGS. 8, 9, 11, and 12.

An example of a control system 6 in a droplet mist generator accordingto the invention can be seen in FIG. 19. As many suitable known controlsystems as desired can be used for the purpose of the present invention.

In an advantageous working form of the invention a frequency generatoris connected at a later point to a MOS-FET circuit, which interrupts thecharging process and thus the deflection process of the piezoelectricflexural element, which occurs through a power supply and a resistance,and discharges the piezoelectric ceramic. In so doing the suddenmovement of the piezoelectric flexural transducer is achieved. In thecharging phase, i.e., for example when moving the piezoelectric flexuraltransducer 4 away from the nozzle area 3, the piezoelectric flexuraltransducer 4 is charged with a resistance of 270 in about 150microseconds to 95% of the power supply voltage. With the rising side ofthe square wave signal of the generator at the gate of the MOS-FET thedischarging occurs through the inner resistance of the FETs. This lastsabout 100 nanoseconds. Due to the mechanical inertia of the actuator,the discharging phase must be extended until the piezoelectric flexuraltransducer 4 slowed by the fluid completes the movement and the dropletis ejected. This is achieved with a standard frequency of 5,000 to 6,000Hz through a pulse-duty factor of 25%, i.e., in a time of 40 to 50microseconds.

What is claimed is:
 1. Droplet mist generator for producing a dropletmist comprising:a pump chamber connected to a fluid reservoir andbounded by a casing wall; a nozzle area constructed in the casing wall,said nozzle area having a plurality of nozzles; a plate-shaped,piezoelectric flexural transducer positioned in the pump chamber andattached so that it forms an overhang and is bendable around atransverse axis that runs transversely to the direction of the overhangfor alternatingly carrying out a displacement stroke, wherein fluid isdriven towards the nozzles of the nozzle area and fluid dropletsproduced are ejected from the nozzles in the form of a droplet mist anda return stroke, whereby the piezoelectric flexural transducer is commonto the plurality of the nozzles of the nozzle area; side openings formedbetween lateral edges of the piezoelectric flexural transducer and aportion of the casing wall lying opposite to said lateral edges, andwherein a connection between the fluid reservoir and the pump chamberempties into the pump chamber at the side of the piezoelectric flexuraltransducer turned away from the nozzle area, and; a control system bywhich the piezoelectric flexural transducer is controlled by voltagepulses for the displacement stroke, which occurs more quickly than thereturn stroke in which the fluid flows back through the side openings.2. Droplet mist generator according to claim 1, whereby the pump chamberis connected to the fluid reservoir through several lines.
 3. Dropletmist generator according to claim 1, whereby the connection between thepump chamber and the fluid reservoir has a choke site.
 4. Droplet mistgenerator according to claim 1, whereby the nozzles are designed totaper in the direction away from the pump chamber.
 5. Droplet mistgenerator according to claim 1, whereby that part of the casing wallconstructed with the nozzle area is covered on outside with Teflon. 6.Droplet mist generator according to claim 1, whereby the piezoelectricflexural transducer is a multi-layer piezoelectric ceramic transducerwith an additional passive piezoelectric ceramic layer.
 7. Droplet mistgenerator according to claim 1, whereby the nozzle area is constructedin a first part of the casing wall that is located under the overhang ofthe piezoelectric flexural transducer in the direction in which a freeend of the piezoelectric flexural transducer can be moved, and a frontalgap is constructed between the free end of the piezoelectric flexuraltransducer and a second part of the casing wall lying opposite to saidfree end.
 8. Droplet mist generator according to claim 7, whereby in anequilibrium position of the piezoelectric flexural transducer, whichoccurs when the voltage is not on, an equilibrium gap is formed betweenthe piezoelectric flexural transducer and that part of the casing wallwhere the nozzle area is constructed, and by applying the voltage, thepiezoelectric flexural transducer can be moved to or from the nozzlearea.
 9. Droplet mist generator according to claim 8, whereby thefrontal gap constructed between the free end of the piezoelectricflexural transducer and the second part of the casing wall lyingopposite to said free end is not more than five times as large as theequilibrium gap.
 10. Droplet mist generator according to claim 9,whereby in the equilibrium position of the piezoelectric flexuraltransducer, which occurs when the voltage is off, the piezoelectricflexural transducer contacts that part of the casing wall where thenozzle area is constructed, and the piezoelectric flexural transducercan be moved away from the nozzle area by applying voltage.
 11. Dropletmist generator according to claim 7, whereby that part of the casingwall where the nozzle area is constructed projects into the pumpchamber.
 12. Droplet mist generator according to claim 7, whereby anarrangement that is essentially mirror-inverted to the piezoelectricflexural transducer and the nozzle area and that has a secondpiezoelectric flexural transducer and a second nozzle area is positionedopposite to the free end of the piezoelectric flexural transducer, andthe control system is constructed so as to control the piezoelectricflexural transducer and the second piezoelectric flexural transducerwith varying pulse frequencies, pulse lengths, and/or pulse phases. 13.Droplet mist generator according to claim 1, whereby the nozzle area ispositioned in that part of the casing wall opposite to a free end of thepiezoelectric flexural transducer.
 14. Droplet mist generator accordingto claim 1, wherein the droplet mist generator is coupled to a burner asa component of the burner, whereby the fluid reservoir is a fluid fuelreservoir, and the nozzles of the nozzle area serve as burner nozzlesand have a smallest diameter of at least 10 μm and at most 100 μm. 15.Droplet mist generator according to claim 14, whereby a distance betweenmid-points of neighboring nozzles of the nozzle area serving as theburner nozzle is at least 50 μm and at most 2,000 μm.
 16. Droplet mistgenerator according to claim 1, which has at least 50 said nozzles.